Organic photoresist strippers are commonly used both for the removal of photoresist from metallized wafers as for the cleaning of the metal layers during the device manufacturing cycle. The most important requirement for such strippers therefore is a negligible metal microcorrosion property. Modern organic strippers, however, are composed of an alkanolamine and a polar organic solvent. When such strippers are rinsed from the wafer surface during processing, the amine group reacts with the water and hydroxide ions are generated.
High alkalinity has a detrimental effect on the exposed metal layers, especially on aluminium and its alloys. Therefore, it is very important that the high alkalinity of the rinsing solution as occurring after solvent stripping is decreased as fast as possible.
To solve this problem, it is common practice to dip the wafers in isopropylalcohol (IPA) or other organic solvents to remove the amine groups of the organic strippers before water contact. Alternatively, organic acidic compounds are added to the photoresist stripper solution as corrosion inhibitors (G. Schwartzkopf, et al (1993) Proceedings of the Symposium on Interconnects, Contact Metallization and Multi-level Metallization and Reliability for Semiconductor Devices, Interconnects and Thin Insulator Materials. ECS Proc. 93-25, p. 379, Electrochemical Society, Pennington, N.J.).
For obvious environmental reasons, e.g. waste disposal problems, there is a wish to decrease the total consumption of organic chemicals. Moreover, by avoiding an organic solvent dip after stripping, there is no need for an intermediate bath and obvious cost savings on room space can be generated. Therefore, the use of inorganic acids to reduce the alkalinity of the post-stripping rinse solution has been proposed. U.S. Pat. No. 5,336,371 and U.S. Pat. No. 5,175,124 describe the use of carbonic acid, a weak inorganic acid, to acidify the rinsing water. This is achieved by bubbling CO.sub.2 through the water-based rinsing solution. In this process monitoring the concentration of the carbonic acid is difficult. In particular, the pH of the carbonated rinse solution will depend upon the total amount of CO.sub.2 that will dissolve in the water. This is influenced by temperature, exposure time to air etc., and therefore difficult to control.
EP 596 515 discloses how metal corrosion is avoided by adding weak acids to alkaline containing strippers. Thereby the nucleophilic amine is partially neutralized by the formation of a reversible salt-like complex. However, during the subsequent rinsing, free amine groups originating both from the non-neutralized fraction and possibly from the dissociated complex fraction, will react with water and thus lead to the formation of hydroxyl ions. Thus, the use of such weak acids makes an accurate pH control difficult resulting in improper neutralization of the rinse solution which can be detrimental to the metal layer.